Phylum Aschelminthes
Phylum Aschelminthes (Gr., askes = cavity ; helmins = worm) is a group of bilaterally symmetrical, unsegmented, and pseudocoelomate animals. They are typically characterized by a cylindrical or thread-like body covered with a tough, flexible cuticle. The phylum includes organisms that are predominantly aquatic or parasitic in nature, such as nematodes (roundworms).
General characteristics of Phylum Aschelminthes
- Bilaterally symmetrical, unsegmented worms.
- Triploblastic and pseudocoelomate animals with organ-system grade of body organization.
- Body usually worm-like cylindrical or flattened.
- Cuticle usually present and cilia absent.
- Alimentary canal straight and complete with mouth and anus.
- Respiratory and circulatory systems absent.
- Excretory system includes a system of canals, protonephridia present in some forms for osmoregulation.
- Nervous system is simple.
- Sense-organs are in the form of pits, papillae, bristles and eye spots.
- Asexual reproduction does not occur.
- Sexes separate, i.e., dioecious and reproductive organs relatively simple.
Classification of Phylum Aschelminthes
* Phylum Aschelminthes is a heterogenous assemblage of different animals. Here only characteristics and classification of Class Nematoda will be described.
CLASS 1. NEMATODA
- Aquatic or terrestrial, free-living or parasitic, elongated roundworms.
- Body wall with cuticle, cellular or syncytial epidermis and longitudinal muscles in four bands.
- No cilia, no circulatory and respiratory systems.
- Digestive system complete with muscular pharynx and glands.
- Excretory system of glandular organs or canals or both.
- Dioecious; male smaller than female.
- Fertilization internal.
Order 1. Enploidea
- Cuticle not ringed, often with cuticular bristles.
- Anterior end with six labial papillae.
- 10 to 12 bristles in one or two circlets.
- Oesophagus is divided into two parts, anterior muscular and posterior glandular.
- A pair of cephalic slits and cyathiform amphids.
- Examples: Enoplus, Anticoma, Synonchus.
Order 2. Dorylaimoidea
- Cuticle smooth without bristles.
- Anterior end with two circlets of papillae of 6 and 10 each.
- Buccal cavity armed with a protrusible spear.
- Rear part of pharynx enlarged.
- Amphids cyathiform.
- Examples: Dorylaimus, Tylencholaimus, Actinolaimus.
Order 3. Mermithoidea
- Smooth, filiform nematodes, parasitic in invertebrates (mainly insects) in juvenile stages or free-living as adults in soil or water.
- Head sense-organs reduced to papillae, usually six.
- Pharynx long, blind; intestine also blind, altered into a food storing organ.
- Amphids cyathiform or reduced.
- Examples: Mermis, Paramermis, Aproctonema.
Order 4. Chromadoroidea
- Aquatic nematodes with smooth or ringed cuticle.
- Usually cuticle heavily ornamented with bristles, knobs, punctations, etc.
- Pharynx with a posterior bulb.
- Amphids spiral or derivable from a spiral.
- Examples: Paracytholaimus, Paracanthonchus.
Order 5. Araeolaimoidea
- Cuticle smooth, sometimes with bristles.
- Labial papillae present.
- Anterior end usually with four cephalic bristles.
- Amphids spiral or loop-like.
- Examples: Plectus, Wilsonema, Odontophora.
Order 6. Monhysteroidea
- Cuticle smooth or slightly ringed, often provided with bristles.
- Anterior end with four, six or eight bristles, or multiples thereof.
- Amphids circular.
- Examples: Cylindrolaimus, Siphonolaimus.
Order 7. Desmoscaleoidea
- Cuticle heavily ringed, with prominent bristles throughout or in restricted areas.
- Anterior end with 4 bristles.
- Amphids crescentic.
- Marine nematodes.
- Examples: Desmoscolex, Tricoma, Greeffiella.
Order 8. Rhabditoidea or Anguiluloidea
- Free-living or parasitic nematodes with ringed or smooth cuticle.
- Pharynx with posterior bulb and also usually with swelling anterior to the nerve ring.
- Caudal glands absent.
- Amphids small pockets.
- Examples: Rhabditis, Diploscapter, Diplogaster.
Order 9. Rhabdiasoidea
- Smooth nematodes with definite pharyngeal bulb.
- Hermaphrodite; parthenogenesis also occurs.
- Parasitic stages in animals.
- Free living stages may develop into males and females.
- Examples: Rhabdias, Entomelas.
Order 10. Oxyuroidea
- Pharynx with a posterior bulb, usually valvulated.
- Females with a long pointed tail; terminal parts of female system often heavily muscularized.
- Males with one spicule or two equal spicules.
- Usually caudal alae forming a cuticular bursa.
- Example: Oxyuris or Enterobius.
Order 11. Ascaroidea
- Mouth surrounded by three prominent lips.
- Pharynx without posterior bulb or if present it is not valvulated.
- Pharynx or intestine or both often with caeca.
- Buccal capsule absent.
- Tail of female blunt; male without caudal alae and possesses two equal or nearly equal spicules.
- Example: Ascaris.
Order 12. Strongyloidea
- Mouth without conspicuous lips but often with leaf crowns.
- Pharynx without bulb.
- Females usually with ovijectors.
- Males with copulatory bursa supported by muscular rays; typically 13 in number.
- Examples: Ancylostoma, Strongylus, Necator.
Order 13. Spiruroidea
- Mouth usually with two lateral lips; some-times 4 or 6 small ones.
- Pharynx without bulb, anteriorly muscular and posteriorly glandular.
- Males without bursa spicules unequal and dissimilar.
- Examples: Thelazia, Rictularia, Oxyspirura.
Order 14. Dracunculoidea
- Without definite lips or cuticularized buccal capsule.
- Pharynx as in Spiruroidea.
- Vulva near or behind the middle of body; usually not functional.
- Males with equal filiform spicules; bursa wanting.
- Examples: Dracunculus, Philometra, Micropleura.
Order 15. Filarioidea
- Filiform worms without lips.
- Buccal capsule small or rudimentary.
- Pharynx as in Spiruroidea, pharyngeal bulb wanting.
- Vulva anterior in female.
- Bursa wanting, spicules as in Spiruroidea.
- Examples: Wuchereria, Loa loa.
Order 16. Trichuroidea or Trichinelloidea
- Body filiform anteriorly.
- Mouth without lips.
- Pharynx slender.
- Provided with a cirrus, spicule if present one only.
- Examples: Trichinella, Trichuris.
Order 17. Dioctophymoidea
- Moderate to very long-sized nematodes.
- Mouth without lips surrounded by 6, 12 or 18 papillae.
- Pharynx elongated without bulb.
- Males with muscular bursa but without rays. Examples:
- Dictyophyme, Hystrichis, Eustronglides.
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